Thermoplastic sheets and films have a broad range of applications. For instance, thermoplastic films and sheets can be found in automotive applications, electronic applications, military applications, appliances, industrial equipment, and furniture.
One important use of films is their use as substrates, or coatings on, flexible circuit applications. In order to serve in this role a new film should meet two requirements critical for flexible circuit substrates, namely low coefficient of thermal expansion (CTE) and high temperature survivability (especially when a high temperature fabrication step is employed).
Low CTE is necessary to match, as closely as possible, the CTE of copper (CTE=17 ppm/° C.). This keeps the film from curling upon temperature change when the film is a substrate for a copper layer, or copper circuit traces. Low CTE also prevents unmatched changes in dimension between the copper and substrate layers upon thermal cycling, which increases the lifetime of the final flexible circuit by reducing stress and fatigue on the patterned copper traces. In other words, the properties of flexible circuit boards are benefited when their film substrate and applied conductive metal layer expand and contract at the same rate. When these layers don't expand and contract at the same rate issues regarding the adhesion, and orientation of the layers can and do arise. While a CTE of less than 70 ppm/° C., specifically less than 60 ppm/° C., even more specifically less than 30 ppm/° C. will allow low warpage upon thermal cycling and is a common goal, better results will be achieved as the CTE of the film becomes closer to the CTE of copper.
CTE is tested by a TMA or Thermomechanical Analysis. The dimension change of a film sample is determined as a function of temperature, and from the slope of this change, the CTE is calculated. Typically, the CTE must be measured for the temperature range that the film is expected to see during flex circuit processing. A temperature range of 20 to 250° C. is a reasonable temperature range for determination of the CTE.
High temperature survivability can also be an important property for the substrate film to survive the soldering process during flex circuit fabrication. The film should exhibit survivability for short periods at elevated temperatures of, for example, 260° C. for new lead-free soldering processes. The standard test for temperature survivability is the solder float test, where a small piece of film is affixed to a cork and is immersed for 10 seconds in molten solder. The film is then removed, the solder is wiped off, and the film is examined. If there is any visible warpage or bubbling, the film fails the test. While there is not a standard thickness for this test, the minimum thickness at which the film passes the solder float test can be reported. Temperatures of 260° C. and 288° C. are standard solder float temperatures for lead-eutectic and lead-free solders, respectively.
Low CTE and high temperature resistance requirements for flexible circuit substrates have been addressed through the use of polyimide films. Many commercial polyimide (PI) films have a high glass transition temperature (greater than 350° C.), and can be partially crosslinked, giving exceptional temperature survivability. The polymer molecules in these films are stressed slightly as they are produced, leading to alignment of the polymer molecules and giving PI films a low CTE. Since the films never see temperatures above the glass transition temperature (Tg) of the material, the stress is never able to relax and the films are dimensionally stable at flex fabrication temperatures.
As thermoplastic sheets and films are used in an increasing wide array of applications the need for thermoplastic sheets and films that can withstand elevated temperatures for appropriate periods of time without substantial degradation is growing. There is a continuing need for films having: a) a CTE under seventy ppm/° C., specifically under thirty ppm/° C. and as close to the CTE of copper as technically possible; and b) high thermal survivability.